HSDPA (high speed data packet access) systems of high speed packet transfer systems have been investigated as transfer systems for down-link channels in third generation mobile communications systems using code division multiple access (CDMA) systems, especially W-CDMA (wideband CDMA) systems.
In such HSDPA systems, code division multiplexing and time division multiplexing are used in combination, and a plurality of time slots are provided for time division multiplexing in the respective down-link channels. Furthermore, data for a mobile station (user) is assigned to each time slot and transmitted.
In the case of such an HSDPA system, several schedulers have been proposed for determining which user's data is assigned to which time slot, i.e., for determining the transmission timing of the user data. For instance, the maximum CIR method, proportional fairness method (maximum fluctuation user selection method) and the like have been proposed (for example, see Patent Reference 1).
The maximum CIR method is a method in which the reception quality values of the respective mobile stations are compared, and data of the mobile station with the best reception quality is given priority in assignment to a time slot. The proportional fairness method is a method in which the data for which the ratio of the mean reception quality value for each mobile station (reception quality averaged over time at each mobile station) to the instantaneous reception quality shows a maximum value is given priority in assignment to a time slot. Thus, in both of these scheduling methods, the assignment of time slots is determined on the basis of reception quality, notification of which is received from the mobile stations.
However, in actual scheduling devices that perform scheduling, a processing delay is generated from the time that the reception quality is acquired from the mobile stations to the time that scheduling is performed and data is assigned to time slots and transmitted. Because of this processing delay, the conditions of the wireless transmission path vary by the time that data is transmitted from the base station to the mobile stations by means of time slots, so that there is a danger that a problem of a drop in throughput due to erroneous assignment of the data transfer rate will arise.
FIG. 14 shows the variation in the conditions of the transfer path according to this processing delay. The horizontal axis indicates time. The vertical axis indicates the information that is provided by the mobile station (mobile station information, reception quality), e.g., the signal-to-interference ratio (SIR) of the mobile station. The mobile station periodically transmits this mobile station information to the base station. The time instant t1 at the point P1 is the time instant at which the mobile station measures its own reception quality (this may be viewed as the time instant at which the wireless packet in the base station is assigned to a time slot). This value S1 is the value of the reception quality at the time instant t1. The time instant t2 at the point P2 is the time instant at which the wireless packed assigned to a time slot by the base station is transmitted to the mobile station; this value S2 is the reception quality at the time instant t2.
Thus, as a result of the time delay d (=t2−t1) between the points P1 and P2, the reception quality at the actual time of transmission of the packet differs from the reception quality at the time of wireless packet assignment. Even if a packet is transmitted to a mobile station selected on the basis of the value of the reception quality at the time instant t1, in cases where the reception quality has dropped at the time of transmission, it may become impossible for the mobile station to receive data, or large numbers of data errors may be generated, so that a large quantity of processing such as the re-sending of packets or the like is necessary. As a result, the throughput drops.
Furthermore, a conventional technique for the selection of the base station by the mobile station is a technique in which the mobile station schedules the measurement of the reception electric field intensity of the signals from a plurality of base stations, and efficiently selects the base station prior to updating (for example, see Patent Reference 1).
Furthermore, a technique relating to a base station device which monitors the transmitting power of a plurality of communications terminals, schedules a shared channel on the basis of the monitoring results, and performs the transmission of this shared channel according to this scheduling, is known as a conventional technique in which the base station device performs MSC selection and scheduling of a shared channel (DSCH) in which a single channel is used by a plurality of communications terminals (for example, see Patent Reference 2).
Non-Patent Reference 1
Shingaku Giho RCS2001-291, pp. 51-58, March 2002 “Characteristic comparison of scheduling methods noting throughput of respective users in down-link high-speed packets”.
Patent Reference 1
Japanese Patent Application Laid-Open No. 2001-157258
Patent Reference 2
Japanese Patent Application Laid-Open No. 2002-290327